Rock bolts are used for ground stabilization, as in the mining industry or they can be used as ground anchoring devices as for anchoring heavy equipment to a ground surface. There are rock bolts known to include a rod threaded at each end, there being an anchor threaded onto one end, the anchor being for embedding in a ground formation. The anchor includes a cone threaded onto the rod, a "C"-shaped shell surrounding the cone and an axial abutment, often provided by a thrust ring, for the shell. Such a rock bolt is described in U.S. Pat. No. 3,837,258, issued Sep. 24, 1974 to Williams, the specification of which is incorporated herein by reference. Williams Form Engineering Corp. currently markets an anchor assembly having a smooth shell design under the trademark "SPIN-LOCK" and claims the anchor has been field proven to far exceed in tension capacity any other mechanical anchor on the market.
In practice, an anchor is typically installed by first drilling a hole into a ground formation. The anchoring end of the bolt is inserted into the hole with the proximal end of the rod projecting out of the hole. The arrangement of the anchor is such that relative rotation of the rod and cone in the appropriate direction results in movement of the cone along the rod to press the shell axially against the abutment and continued movement of the cone toward the abutment and against the shell causes the shell to expand. Ultimately the shell becomes jammed against the side of the hole so as to set the anchor in the hole. An apertured face plate is located against the ground facing around the hole with the proximal end of the rod projecting therethrough and a nut is threaded along the rod against the face plate to stress the rod as desired.
It is often desirable to infuse grout into the hole once a rock bolt has been installed. To this end, rods of some rock bolts in common use today are hollow. In installations, such as underground mines, where the hole is oriented upwardly into the roof of the mine, grout is generally pumped through an opening in the face plate to fill the hole and surround the bolt rod. Eventually the grout reaches the top of the rod and travels back down the center hole of the rod to emerge from the rod and give an indication of the completion of the grouting operation. In installations where a hole is oriented downwardly, the grout is generally injected through the rod and eventually emerges around the face plate to give an indication of the completion of the grouting operation.
A weak link in such a bolting arrangement often turns out to be the anchoring portion of the installation. It has been found that, over time, with stresses placed on the installation by ground movement, etc., the anchor portion does not sustain the tensile or pull-out forces placed on it and slippage occurs between the anchor shell and wall of the hole against which the shell is jammed.
This problem has not gone unrecognized, but an entirely satisfactory solution has apparently not been found. For example, the Williams patent specification, mentioned above, discloses cylindrical anchor shells having a circular outer cross section, that is, a relatively smooth rock contact area. The specification also describes a modified anchor shell having an arcuate depression (preferrably annular) and a modified anchor shell having notch-shaped irregularities (preferrably annular). These modified forms of anchor shell are described as facilitating the development of frictional and pressure-centered retaining forces.
The precise causes of difficulties that arise in the use of various rock bolt shells are not entirely understood, but current field practice is to choose a particular type of shell according to the type of rock formation in which the bolt is to be installed. Smooth shells are generally used in preference to ridged shells in conjunction with soft rock formations. The compressive strength of rock is generally about one third to one half greater than the shear strength of rock. A smooth-walled shell in which large surface area contact between the shell and rock is provided transfers a greater proportion of load to surrounding rock through compression than a ridged shell. Because of reduced shearing forces, a smooth-walled shell is thus expected to result in reduced localized crushing of soft rock relative to a ridged shell.
On the other hand, the problem of localized crushing of rock by shear forces is expected to be reduced when a bolt is anchored in relatively hard rock. In the case of a ridged shell, in which point contact between the shell and surrounding rock increases shear load transfer forces:, an enhanced resistance to slippage is thus expected.
There are also, at times, difficulties in setting a rock bolt in its hole. This may be due in part to a spinning of the entire bolt within its hole during the setting operation described above.
In any event, it would be advantageous to have a single rock bolt shell which could be used in a wider variety of rock formations than any shell currently available. At the very least, a rock bolt having enhanced performance characteristics in a particular context, stemming from either or both enhanced setting or anchoring characteristics, would be considered an improvement.